Merge pull request #196 from 30-seconds/covid-snippets

Add new snippets

snippets/collect_dictionary.md

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+---
+title: collect_dictionary
+tags: dictionary,intermediate
+---
+
+Inverts a dictionary with non-unique hashable values.
+
+Use `dictionary.items()` in combination with a loop to map the values of the dictionary to keys using `dictionary.setdefault()`, `list()` and `append()` to create a list for each one.
+
+```py
+def collect_dictionary(obj):
+  inv_obj = {}
+  for key, value in obj.items():
+    inv_obj.setdefault(value, list()).append(key)
+  return inv_obj
+```
+
+```py
+ages = {
+  "Peter": 10,
+  "Isabel": 10,
+  "Anna": 9,
+}
+collect_dictionary(ages) # { 10: ["Peter", "Isabel"], 9: ["Anna"] }
+```

snippets/find_key.md

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+---
+title: find_key
+tags: dictionary,intermediate
+---
+
+Returns the first key in the provided dictionary that has the given value.
+
+Use `dictionary.items()` and `next()` to return the first key that has a value equal to `val`.
+
+```py
+def find_key(dict, val):
+  return next(key for key, value in dict.items() if value == val)
+```
+
+```py
+ages = {
+  "Peter": 10,
+  "Isabel": 11,
+  "Anna": 9,
+}
+find_key(ages, 11) # "Isabel"
+```

snippets/find_keys.md

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+---
+title: find_keys
+tags: dictionary,intermediate
+---
+
+Returns all keys in the provided dictionary that have the given value.
+
+Use `dictionary.items()`, a generator and `list()` to return all keys that have a value equal to `val`.
+
+```py
+def find_keys(dict, val):
+  return list(key for key, value in dict.items() if value == val)
+```
+
+```py
+ages = {
+  "Peter": 10,
+  "Isabel": 11,
+  "Anna": 10,
+}
+find_keys(ages, 10) # [ "Peter", "Anna" ]
+```

snippets/invert_dictionary.md

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+---
+title: invert_dictionary
+tags: dictionary,intermediate
+---
+
+Inverts a dictionary with unique hashable values.
+
+Use `dictionary.items()` in combination with a list comprehension to create a new dictionary with the values and keys inverted.
+
+```py
+def invert_dictionary(obj):
+  return { value: key for key, value in obj.items() }
+```
+
+```py
+ages = {
+  "Peter": 10,
+  "Isabel": 11,
+  "Anna": 9,
+}
+invert_dictionary(ages) # { 10: "Peter", 11 "Isabel", 9: "Anna" }
+```

snippets/merge_dictionaries.md

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+---
+title: merge_dictionaries
+tags: dictionary,intermediate
+---
+
+Merges two or more dictionaries.
+
+Create a new `dict()` and loop over `dicts`, using `dictionary.update()` to add the key-value pairs from each one to the result.
+
+```py
+def merge_dictionaries(*dicts):
+  res = dict()
+  for d in dicts:
+    res.update(d)
+  return res
+```
+
+```py
+ages_one = {
+  "Peter": 10,
+  "Isabel": 11,
+}
+ages_two = {
+  "Anna": 9
+}
+merge_dictionaries(ages_one, ages_two) # { "Peter": 10, "Isabel": 11, "Anna": 9 }
+```

snippets/to_dictionary.md

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+---
+title: to_dictionary
+tags: list,dictionary,intermediate
+---
+
+Combines two lists into a dictionary, where the elements of the first one serve as the keys and the elements of the second one serve as the values.
+The values of the first list need to be unique and hashable.
+
+Use `zip()` in combination with a list comprehension to combine the values of the two lists, based on their positions.
+
+```py
+def to_dictionary(keys, values):
+  return {key:value for key, value in zip(keys, values)}
+```
+
+```py
+to_dictionary(['a', 'b'], [1, 2]) # { a: 1, b: 2 }
+```